Silyl-containing substituted polyacetylenes have been noted to exhibit the highest gas permeability of any known polymeric materials (T. Masuda and T. Higashimura, Adv. Polym. Sci., 81, 121 (1987)). Poly(trimethylsilylpropyne), for example, has been shown to exhibit an O.sub.2 permeability of about 10 times that of silicone rubber. Prior to the appearance of poly(trimethylsilylpropyne) permeability data, silicone rubber was noted to be the most permeable polymeric material known. While the silyl-containing substituted polyacetylenes exhibit very high gas permeability, their selectivity (e.g., O.sub.2 /N.sub.2) for gas separation is quite low. Generally, for applications involving enhanced oxygen (e.g., for enhanced combustion applications) separation factors of 2.0 or greater are desired. Poly(trimethylsilylpropyne) has been reported to have an O.sub.2 /N.sub.2 separation factor of 1.8 (Isobe et al., J. Polym. Sci.: Part A: Polym. Chem., 24, 1839 (1986)). Attempted duplication of this by the Applicants. however, indicated that the O.sub.2 /N.sub.2 separation factor for ths polymer was in a range of about 1.4 to 1.5. One possible route to solving this problem would be to prepare silyl-containing substituted polyacetylenes of different structure. Several examples of this exist, such as poly(1-dimethyl-nhexylsilyl)-1-propyne) and poly(4,4,6.6-tetramethyl-4,6-disila-2-heptyne) which have higher separation factors for O.sub.2 /N.sub.2 than poly(trimethylsilylpropyne) and also lower O.sub.2 permeabilities (see Isobe et al., above).
While this is a possible approach, another facile method may be to add a material to poly(trimethylsilylpropyne) which yields the same end result (e.g., increasing the separation factor while maintaining reasonably high permeabilities). It has been found that the addition of certain classes of low molecular weight liquids which exhibit miscibility with poly(trimethylsilylpropyne) and have very low volatility yield improved separation factors to O.sub.2 /N.sub.2 separation.
In many cases, the addition of low molecular weight liquids to polymeric materials results in a lowering of the modulus (increased flexibility) and increased permeability of gases as the molecular chains exhibit more mobility. There are a number of examples which include the following: Felder and Huvard (Methods of Experimental Physics. 16C, 315(1980)) note "the introduction of a plasticizer into a polymer either during fabrication or subsequently by permeation increases the mobility of chain segments and consequently increases the effective diffusion coefficient, primarily by lowering the activation energy of diffusion." Stannett (in "Diffusion in Polymer", ed. by J. Crank and G. S. Park, p. 62-64 , Academic Press, New York, 1968) noted "the addition of a plasticizer polymer decreases the cohesive forces between the chains resulting in an increase in segmental mobility. It is clear that this should result in an increased rate of diffusion and a lower activation energy." Brubaker and Kammermeyer in an early study (Ind. Eng. Chem., 45. 1148(1953)) noted increases in gas permeation with plasticizer additon to poly(chlorotrifluoroethylene) and cellulose acetate. Brown and Sauber (in Modern Plastics, August 1959) noted that plasticized poly(vinlychloride) exhibits much higher O.sub.2, N.sub.2, and CO.sub.2 permeabilities than rigid (unplasticized) poly(vinylchloride). While most of the data in the literature suggests that low molecular weight liquids will result in reduced modulus and increased permeability. there are examples whereby the addition of low molecular weight liquids (primarily those with rigid units such as aromatic groups) to glassy polymers having large secondary loss transitions lead to a reduction in permeability and an increase in modulus (see L. M. Robeson, Polym. Eng. Sci., 9, 277 (1969)). Polymers which exhibit large secondary loss transitions include polycarbonates, polysulfones, polyhydroxyethers, poly(aryletherketones), aromatic polyesters (e.g., PET), and polyarylates. Poly(trimethylsilylpropyne) and other silyl substituted polyacetylenes we have investigated show no significant secondary loss transitions, and thus would not be expected to be capable of antiplasticization. More recently, work reported by Maeda and Paul (J. Mem. Sc., 30, 1(1987)) noted antiplasticized polymers yielded increasing separation factors for several gas pairs (e.g., He/CH.sub.4, He/N.sub.2, H.sub.2 /CH.sub.4). Poly(trimethylsilyl- propyne), however, has been found to exhibit behavior different than either plasticzed or antiplasticized polymers. As an example, silicone oil addition to PTMSP leads to no change in mechanical properties, but decreased permeability. This behavior appears to be unique for poly(trimethylsilylpropyne) and other silyl substituted polyacetylenes. The separation factor for O.sub.2 /N.sub.2 increases with addition of silicone oil (as well as other miscible liquids) and is the essence of this invention.
Poly(trimethylsilylpropyne) has been noted to exhibit a decreasing permeability with time. Nakanishi. et al., noted (Polym. J., 19, 293 (1987)) that the gas permeability of PTMSP decreased with time under vacuum. Asakawara, et al. (Japanese Patent Disclosure No. 61-35823, Feb. 20. 1986) noted an initial P(O.sub.2) of 7000 barrers for PTMSP when cast from toluene. Casting from benzene and spread on a water surface or heat treatment at 50.degree. C. yielded a remarkable change in permeability (decreasing to 200 barrers). Asakawara, et al., found a lower alcohol treatment of the heat aged film or benzene cast film returned the permeability values back to the original "intrinsic" value. Masuda, et al. (J. Appl. Polym. Sci., 30, 1605 (1985)) reported the P(O.sub.2) value for PTMSP decreased to 1/10th of the original value when heated at 100.degree. C. for 15 hours. Masuda, et al. (Adv. Polym. Sci., 81, 121 (1987)) reported the "P(O.sub.2) value of PTMSP gradually decreases to about 1% of its original value when the membrane is left at room temperature for several months." They suggested that the "membrane of PTMSP has many molecular-scale holes just after its preparation, while relaxation of the structure occurs with time to make the holes smaller and fewer." This problem of aging which has been well-documented in the open literature raises serious questions about the utility of PTMSP and thus other silyl substituted polyacetylenes for gas (in particular O.sub.2 /N.sub.2) separation applications. Nakagawa (Japanese High Technology Monitor, 4(11), June 5, 1986) reported that PTMSP has initial high gas permeability which decreases rapidly with time. Nakagawa noted that addition of 3-5% dioctyl phthalate followed by heat treatment of the membrane yields a stable gas permeability with an O.sub.2 /N.sub.2 separation factor of about 3. In the studies reported herein, we have not been able to duplicate the results of the above aging problem except in a case of films exposed to vacuum for extended periods. Normal casting and permeability testing conditions in our experiments demonstrate a remarkable P(O.sub.2) stability versus time for PTMSP even approaching a year duration. We have also found that it is not necessary to heat treat membranes to obtain stable films or is it necessary to add additives followed by heat treatment to achieve stable P(O.sub.2) values. In fact, we have found prior to the above-stated references that PTMSP exhibits stable P(O.sub.2) values and addition of liquids (e.g., oils such as Nujol oil and silicone oil) lead to reduced O.sub.2 permeability and increased O.sub.2 /N.sub.2 separation factors which are stable without further heat treatment. We have also found that heat treatment (without vacuum) for extended periods does not lead to the dramatic reductions in permeability reported in the literature. The conclusion reached in consideration of all this data is that the results reported in the literature are based on the exposure of PTMSP films to vacuum resulting in "aerosol" vacuum oil being sorbed by the PTMSP films resulting in lowered O.sub.2 permeability thus analogous to the experiments where silicone oil or Nujol oil have been purposely added. It is much preferred to add the oil or liquid initially to the film instead of exposure to vacuum to yield the end result. Our conclusions on vacuum oil sorption were recently confirmed by Witchey, et al. (paper titled "Sorption and Transport of Organic Vapors in Poly(1-trimethylsilyl-1-propyne", presented at the Annual Mtg. of the AICHE, New York City, Nov. 16. 1987) where it was noted that PTMSP exposed to 10.sup.-3 to 10.sup.-4 Torr vacuum increased in weight and yellowed after extended periods of exposure and, upon removal, exuded the pump oil odor. This result previously verified with P(O.sub.2) experiments in our own laboratories is thus unexpected and removes the serious questions noted in the literature concerning the applicability of PTMSP and other silyl substituted polyacetylenes for gas separation applications. In fact. direct addition of certain liquids and oils (including mechanical pump oils) leads to an improved separation capability and does not require heat treatment for stability. This observation, combined with the unique properties of PTMSP which are unlike typical plasticized or antiplasticized polymers with miscible oil or liquid addition, is unexpected and is the essence of the invention clamed herein.
Other modifications of silyl-containing substituted polyacetylenes to yield improved separation factors for O.sub.2 /N.sub.2 as well as other gas pairs of interest include surface fluorination (M. Langsam, U.S. Pat. No. 4.657,564, Apr. 14, 1987), plasma treatment (Nomura, et al., U.S. Pat. No. 4,607,088, Aug 19, 1986). and addition of Fe phthalocyanine tetracarboxylic acid (Asakawara, et al., Jpn. Kokai Tokyo Koho JP 62133526 A2 (87/33526), Feb. 13, 1987).
The utility of the additive modified silyl substituted polyacetylenes lies primarily in the use as a permeable membrane for gas separation in general with particular interest in O.sub.2 /N.sub.2 separation. This utility is of particular interest for oxygen enriched air applications which are important in energy savings in combustion use (including oxygen enrichment for industrial combustion processes, domestic heating use, as well as for internal combustion engine efficiency improvement for transportation), enriched oxygen for medical use, enriched oxygen for sludge treatment, enriched oxygen for hybrid air separation processes to improve efficiency of cryogenic or adsorption processes.
Membrane separation by dfferential gas permeabilities is an emerging technology which has reached commercial status in the past decade and continues to grow in importance. For enhanced oxygen applications, silicone rubber has been considered to be the primary membrane of choice. Silicone rubber has a very low modulus (thus poor load or pressure bearing capabilities), must be crosslinked for utility, and is difficult to prepare in ultra-thin membranes. Silyl substituted polyacetylenes offer much higher modulus (&gt;100 times that of silicone rubber), do not need crosslinking, and can be prepared in ultra-thin membranes easily. Certain membranes of the silyl substituted polyacetylene family (e.g. poly(trimethylsilylpropyne) offer much higher permeability than silicone rubber.